On the other hand, the tau modes \(\bar{B} \rightarrow {D^{(*)}}\tau \bar{\nu }\)  are of particular interest
because of the excesses in the experimental measurements of \(R_{{D^{(*)}}} = \mathcal {B} (\bar{B} \rightarrow {D^{(*)}}\tau \bar{\nu }) / \mathcal {B} (\bar{B} \rightarrow {D^{(*)}}\ell \bar{\nu })\)  compared with the SM predictions.
The current measurements are summarized as \(R_{\scalebox {0.7}{D}}^\text{\scalebox {0.7}{exp:WA}} = 0.340(27)(13) \)  and \(R_{\scalebox {0.7}{D^*}}^\text{\scalebox {0.7}{exp:WA}} = 0.295(11)(8)\) 
while several SM predictions are reported, e.g., \(R_{\scalebox {0.7}{D}}^\text{\scalebox {0.7}{SM}} = \lbrace 0.299(4),\, 0.297(6),\, 0.289(4)\rbrace \)  and \(R_{\scalebox {0.7}{D^*}}^\text{\scalebox {0.7}{SM}} = \lbrace 0.258(5),\, 0.245(4),\, 0.248(1)\rbrace \) ,
where the first ones are from HFLAV [1]} and the latter two are from Ref. [2]} based on two different sets for the form factors.
Then, many studies point out that the excesses can be explained by several types of NP currents in \(b \rightarrow c\tau \nu \)  with \(C_\text{NP} \sim \mathcal {O} (10\%)\) .
There also exists the NP study for \(b \rightarrow u\tau \nu \)  as in Ref. [3]}, and then the typical size of the constraint is \(\mathcal {O} (10\%)\)  as well.
